(1) Field of the Invention
This invention relates to an electrical insulating oil and oil-filled electrical appliances which is impregnated with the same.
More particularly, the invention relates to a new electrical insulating oil and oil-filled electrical appliances that are impregnated with the same, where the electrical insulating oil comprises a mixture of dibenzylbenzene isomers. The electrical insulating oil of the present invention is characterized in that both the viscosity and pour point thereof are low, the impregnating property to plastic materials as insulating or dielectric material is good, and it does neither dissolve nor swell the plastic materials.
(2) Description of the Prior Art
In the conventional art, the use of dibenzyltoluene as electrical insulating oil was proposed in Japanese Patent Publication No. 49-14320. However, any instance has never been known yet to those skilled in the art that dibenzylbenzene is used as an electrical insulating oil.
Meanwhile, in recent years, electrical appliances such as oil-filled capacitors have been made small in size, light in weight and durable to high voltages. With this tendency, various kinds of plastic materials have been used singly or together with the conventional insulating paper as insulating or dielectric materials.
The plastic materials will be still more used as the component materials of oil-filled electric capacitors. In some instances, all of the conventional insulating paper are replaced with plastic materials. As the electrical insulating oil used for the electrical appliances in which a plastic material is used, the insulating oil must have compatibility with the plastic material. In other words, when an electrical insulating oil comes into contact with a plastic material, if the oil dissolves or swells the plastic material, the dielectric strength of electrical appliances will be impaired. In the case that the viscosity of an electrical insulating oil is too high, the dielectric strength is also lowered owing to insufficient impregnation. Accordingly, it is necessary for the electrical insulating oil that it is well compatible with plastic materials and the viscosity of the insulating oil is sufficiently low.
Meanwhile, metallized film capacitors (hereinafter referred to as "MF capacitors") are known in the related technical field. In this MF capacitor, a metal-deposited film that is made by vacuum-depositing a metal such as aluminum or zinc is wound as an electrode. The MF capacitors of this kind are used widely because they have self healing action and can be made small in size with high dielectric strength even when they are not provided with any insulating film or paper interposed among layers of electrodes. Furthermore, biaxially stretched polypropylene film is widely used as the base film for these MF capacitors because it is inexpensive as compared with other films such as polyester film, in addition, the temperature characteristic in dielectric loss can be made good.
The MF capacitors presently used are mainly the so-called dry-type ones in which any electrically insulative impregnating agent such as insulating oil is not used.
Not only in electric capacitors but also in other various electrical appliances, the potential gradient can be generally made high when electrodes or electric conductors are wholly surrounded by an electrically insulative impregnatihg agent. That is, it is advantageous because the dielectric strength of electrical appliances can be improved. Furthermore, if an impregnating agent is properly selected, the high-voltage withstanding property can be improved moreover.
Accordingly, oil-impregnated or oil-filled MF capacitors are more preferable than dry-type capacitors. However, when a metal-deposited film with a polypropylene base film is impregnated with an electrically insulative impregnating agent, the sizes of the film are changed or the impregnating agent permeates through the boundary between the base film and the vacuum-deposited metallic layer. Accordingly, the metallic layer is cracked, and what is worse, the metallic layer peels off resulting into dielectric breakdown. In addition, as the capacitor is impregnated with an impregnating agent after the film is wound, if the selection of the impregnating agent is not adequately done, the impregnating agent cannot penetrate satisfactory into the clearances among film layers, from which aimed effect of impregnation cannot be expected.
In the dry-type capacitors, the dielectric breakdown is often liable to occur in the marginal edges or peripheries of wound metallic layers. So that, the dry-type MF capacitors is improved to some degree by modifying them into the so-called semi-dry MF capacitors, by impregnating the edge and peripheral portions with an impregnating agent with leaving internal portions unimpregnated.
There is, however, a limit in the improvement effect in the semi-dry MF capacitors of the above type because it is impregnated only partially and most portions of electrode layers are left unimpregnated, and it may not be denied that the semi-dry MF capacitors are not satisfactory as compared with fully impregnated types. Accordingly, various MF capacitors of impregnated type have been proposed.
For example, proposed in Japanese Laid-Open Patent Publication No. 55-36972 is a impregnated MF capacitor in which the expansion rate of metallized film that is swollen by an impregnating agent is made 0.5% or less. In British Pat. No. 1,451,499, an oil-filled capacitor is disclosed in which the percentage of change in length of polypropylene film owing to an insulating oil is not more than 0.5% at 80.degree. C. and the diffusion quantity of the insulating oil into the polypropylene film is not more than 10% at 100.degree. C., and this capacitor can be an MF capacitor.
However, the above-described MF capacitors do not always fit for practical uses.